Methods for treating polymeric plastic surfaces



y Z, F.J. BOURBEAU ETAL 2,944,294

METHODS FOR TREATING POLYMERIC PLASTIC SURFACES Filed July 1, 1957 INV NTORS BY M ATTORNEYS limited States Filed'July 1, 1957, Sen No. 669,064 scams. (Cl. 18-56) i This invention relates to improved methods for pol? ishing polymeric plastic films to be used; forexample, in connectionwit-h sheetsintended' to provide a base for aphotographicimage-carryinglayer.

. It. has long been. recognizedv that certain linear polymeric. plastic. materials, such,. for examplegascellulose acetate, are: useful in connection with the making ofv pho- EQQ a linear polymer, such as Cellulose acetate, to a base paper such as baryta stock, drying the coating to remove as" much of the solvent as is practicable, and feeding the coated stock to a smooth-surfaced polishing drum, si'mul taneously applying at the nip a minute quantity of a 7 liquid; which has the property, in the quantity and duratographic. prints..- In. the well-Known. Land. diffusiontransfer process, for example, polymeric plastic mate: rials, such as cellulose. acetate, provide. a highly satisfac: tory base layerv for subsequent layers. upon which silver is precipitated. toform a positive photographic.- image. It is desirable, in connection with, such. processes, that uniformity andhigh. gloss be imparted. to. that surface ofisuch, a polymeric plastic base layer" to. which suchsub: sequent layers are applied;

To obtain these properties, it is customary. to deposit a coating of cellulose acetate or a similar,- material upon one surface of a paper stock and then to polish thee posed surface of the coating; this is commonlydone by bringing the coated stock. into contact a smooth, hard surface such, for example, as-a chrome-plated drum. Conventionally, there are limits to the speedand ease. of polishing such, surfaces. For example,- if a conventional procedure is. followed whereby athin: coating of cellulose acetate. is applied to they base paper and, after somev solvents havebeen removed, the coating. is polished for improved: smoothness and gloss, satisfactory polishing will be obtained only if the partly dried coating comes in contact for a relativelylong period with, a heated, polished chrome drum. This requirement for extended. contact necessarily reduces the. rate. at which coated stock. may be polished; this may well be the bottleneck: in. an en:- tire production operation;

Accordingly, the objects of the present invention are to provide. methods for improving the gloss and uni formity of plastic sheets and coatings; to permit; production of high-gloss coated:- stock at high speeds; and to provide a uniform, high-gloss base layer for receiving photographic images printed by means of the Land diffusion-transfer process.

Other objects of the invention will in part by obvione and will in part appear hereinafter.

The invention accordingly comprises the process involving the several steps and the relation andorder of one or more of such steps with respect to each of the others which are exemplified in. the following detailed disclosure, and the scope of the application of which will: be indicated in the claims.

For a fuller understanding of. the nature and objects of thein vention, reference shouid be had to the following detailed description taken in connection with the accompanying drawing which is a schematic representation of" a preferred apparatus for practicing the invention, showing also the arrangement-of steps in the preferred process.

A preferred embodiment of the improved process herein described consists essentially in applying a coating of ion. of application, of softening but not completely dissolving the cellulose acetate coating. The liquid is pref.- erably so selected that, in the amount applied and for the interval during: which it is in contact with the surface to be polished, at least the skin portionof, the cellulose acetate layer is softened. Water, for example, may be employed as a softener forv cellulose acetate,.butsince water has. a. relatively slow softening action, it is desirable to. maintainthe. surface to be. polished in contact with the polishing drumfor, a relatively longer period than would bedesirable for a. liquid which softens at a more rapid rate, such, forexample, as a, methanol-water mixture.

Referring. now to the. schematic representation. in. the drawing, the preferred. apparatus may comprise a supply reel 1 upon which coatedand dried stock has beenwound, spatially separated from a parallel tension. roller 2 which. is in: turn disposed directly below a nip roller 3, Niproller 3 is preferablyprovidcd with a nonmetallic, slightly yieldable surface. and may be sodisposed asv to press the stock against. a polishing drum 14. Above theniproller=3- is positioned reservoir 4, which is of any suitable configuration to hold a desired amount of the softening liquid.v 'Thebase of reservoir 4 is provided with. anopening; 5, leading to a manifold 6. --A suitable feed controlvalve 8 may be provided in the line tomani-i fold 6- or at any'otherv point in the feed assembly. Manifold: 6 may have. a series of exit apertures each connectingwitha feed tubev 10 leading to the nip 12. Preferably, feed tubes 10 should be so spaced laterally that a small, continuous head of liquid will format the. nip; a lateral spacing of the. order of six inches is generally suitable.

Contacting nip roller 3 is a large polishing drum 14; Drum 14. preferably has a cylindrical, polished chromeplated surface which is as free as possible: from scratches, projections or other irregularities. Drum 14 may have provision for heating the polished surface from within by means, for example, of an annular jacket and suitable supply and return passages for passing hot water through the jacket. Provision may be made for varying the pres! sure of niproller 3 against drum '14-.

Directly adjacent to drum 14 is positioned tension roller'I6, which is. in turn spaced a suitable distance from an end storage reel 18.

The process is preferably performed in the following manner. Paper stock such as baryta paper is coated by any suitable means with a thin layer of cellulose acetate which maybe applied in the form of a solution madevup of celluloseacetate. flake dissolved in suitable solvents. For example, a suitable solution may comprise 25 g. cellulose acetate, 180 cc. ethyl acetate and 60' cc. methanol. The coated stock is then dried to remove asmuch of-the solvents as feasible, preferably until less" than the order of" 5% of the original solvent remains. Preferably, the stock should be dry enough to be rollable Without blocke ing. The coated stock may then be wound on a suitable corewhich may then be positioned as supply reel ,1. From reel 1,the coated and dried stock preferably passesunder tension roller 2', around nip roller 3 and is: pressed into contact'with drum 14. A minute quantity of a liquid is applied'to'the coated surface just as it comes into cone tact with drum 14'. This liquid, supplied from reservoir 4 through manifold 6 and feed tubes 10, preferably comprises amixture of methanol and 20% watenzby weight. The methanol-water mixture may preferably be regulated so that two to three drops per second are applied I from each feed tube 10. The rate of flow should be such that only a small, continuous bead of Water-methanol can be seen at the nip. The coated stock is then maintained in contact with the polishing drum 14 for a time on the order of one second. The drum surface may preferably be at a temperature of about 180 F., although temperatures on the order of 30 to 40 higher and 50 to 60" lower do not lead to significantly different results. The pressure of the stock against the drum 14 is not critical, so long as at least a minimal pressure is maintained; a good polish will result at pressures ranging from to 250 pounds per linear inch of stock width, or at even higher pressures.

The stock is stripped from drum 14 by means of tension roller 16 and may then be wound around storage reel 18 for further processing.

Tests have indicated that baryta paper coated with cellulose acetate and treated on the apparatus and in accordance with the process described above may be polished to a high, uniform gloss at speeds of the order of 100 linear feet per minute. Conversely, slower speeds yield an unusually high uniform gloss.

Several alternative arrangements and procedures are possible without departing from the scope of the invention. For example, it is to be understood that the process described above may be incorporated with other earlier and later processes in an assembly line technique by eliminating reels 1 and 18 and instead feeding the stock continuously from or to other equipment.

The apparatus may also be varied by replacing reservoir 4 and its associated feeding system with suitable devices for spraying or otherwise depositing the water-methanol mix upon the cellulose acetate coating at the nip.

Polishing drum 14 may have any suitable internal heating arrangement or may be heated externally. As has been previously indicated, the drum surface temperature may be varied widely. Preferably, the temperature need only be high enough to assist in softening the surface to be polished and to cause substantial redrying of that surface before it is separated from the drum surface. The upper temperature limit is similarly flexible; the'drum temperature should not be so high as to cause the softening liquid to boil away before it has performed its softening function, nor should the temperature be high enough to cause the plastic surface to become overly tacky or to develop other unfavorable properties. The drum surface may be chromium or a chrome alloy, stainless steel, glass, or any other material that will impart the desired surface characteristics to the stock. Although it has been found convenient to use a polishing drum having a diameter between 3 and 6 feet, larger or smaller drums may be used.

A wide variety of materials may be used in the process. For example, other papers or cloths may be used as the base material instead of baryta paper, or one surface of a preformed plastic film may be polished in this manner. Any thermoplastic material which may be softened by a suitable selection of softening agents and heating conditions may be employed. For example, ethyl cellulose or polyvinyl chloride may similarly be polished.

Similarly, a great number of surface-softening materials may be used instead of the water-alcohol mixture. Water,

methanol, ethanol and butanol may be used with cellulose acetate at relatively low processing speeds; mixtures of water and other compounds, such as ethanol-Water, propanol-water or an acetone-water mixture will be effective even at higher processing speeds, i.e., on the order of clude solvents as well as substances which swell or soften the polymeric plastic to which they are applied without actually dissolving it.

It is probable that the increased polishing rate permitted by the present invention is based on the following phenomena:

In conventional polishing, it .is customary to apply the coated stock to a large-diameter, slowly-rotating polishing drum while a substantial percentage of solvent remains in the coating. In this condition, the coating must remain in contact with the drum for a relatively long period of time to cause the entire coating thickness to be hardened. In the present invention, the coating is fully dried and hardened beforehand. The subsequent application of the softening agent at the nip probably penetrates and softens only an extremely thin skin of the coating.

Thus, this small skin may be smoothed and dried to a high polish during a relatively short contact with the polishing drum. As a result, a high gloss may be obtained at significantly higher linear speeds.

Although a prime object of the present invention is to provide an optically smooth, high-gloss surface upon a polymeric plastic material, it is obvious that other surface characteristics may be imparted by substituting, for the smooth polishing drum of the preferred embodiment, a surface having other characteristics such as, for example, lenticules or a pebble, wrinkle, or sand finish. The terms polishing elemen and polishing surfacefas used herein shall be understood as including not only smooth sur-' faced elements but also those having the other indicated surface characteristics.

Since certain changes may be made in the above process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawing, shall be interpreted as illustrative and not in a limiting.

sense.

What is claimed is:

1. The method of polishing the surface of a substantially solvent-free, hardened thermoplastic polymeric layer carried upon a paper support to a high gloss which comprises the steps of providing at least one source of a softening agent for said polymeric layer, moving said polymeric layer past said source of softening agent, contacting said polymeric layer during movement past said source with a quantity of softening'agent suflicient to soften only a thin surface portion of said polymeric layer, substantially immediately after passing said source pressing the softened layer surface into contact with a rotating smooth-surfaced polishing element, said polishing element being heated to a temperature sufiicient to substantially redry said softened layer surface, and maintaining said softened layer surface in contact with said heated, rotating polishing element until a smooth, hard, high-gloss surface is formed.

2. The method of claim 1 wherein said support is baryta paper and said polymeric layer is cellulose acetate.

3. The method of claim 1 wherein said softening agent comprises a solution of water and alcohol.

4. The method of claim 3 wherein said softening agent comprises a solution of about percent by weight of methanol and about 20 percent by weight of water.

References Cited in the file of this patent UNITED STATES PATENTS 1,860,456 Dreyfus -4 May 31, 1932 1,889,045 Dreyfus Nov. 29, 1932 1,889,061 Dickie et al Nov. 29, 1932 1,912,625 Dreyfus June 6, 1933 1,964,724 Dreyfus July 3, 1934 2,218,227 Winnek Oct. 15, 1940 2,296,804 Winnek Sept. 22, 1942 2,562,078 Winnek July 24, 1951 

1. THE METHOD OF POLISHING THE SURFACE OF A SUBSTANTIALLY SOLVENT-FREE, HARDENED THERMOPLASTIC POLYMERIC LAYER CARRIED UPON A PAPER SUPPORT TO A HIGH GLOSS WHICH COMPRISES THE STEPS OF PROVIDING AT LEAST ONE SOURCE OF A SOFTENING AGENT FOR SAID POLYMERIC LAYER, MOVING SAID POLYMERIC LAYER PAST SAID SOURCE OF SOFTENING AGENT, CONTACTING SAID POLYMERIC LAYER DURING MOVEMENT PAST SAID SOURCE WITH A QUANTITY OF SOFTENING AGENT SUFFICIENT TO SOFTEN ONLY A THIN SURFACE PORTION OF SAID POLYMERIC LAYER, SUBSTANTIALLY IMMEDIATELY AFTER PASSING SAID SOURCE PRESSING THE SOFTENED LAYER SURFACE INTO CONTACT WITH A ROTATING SMOOTH-SURFACED POLISHING ELEMENT, SAID POLISHING ELEMENT BEING HEATED TO A TEMPERATURE SUFFICIENT TO SUBSTANTIALLY REDRY SAID SOFTENED LAYER SURFACE, AND MAINTAINING SAID SOFTENED LAYER SURFACE IN CONTACT WITH SAID HEATED, ROTATING POLISHING ELEMENT UNTIL A SMOOTH, HARD, HIGH-GLOSS SURFACE IS FORMED. 